X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-10-02
2004-07-13
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000, C378S146000
Reexamination Certificate
active
06763081
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to 3-dimensional (3D) computerized tomography (CT) and, more specifically, to an improved cone beam CT system and method for efficiently acquiring projection data for image reconstruction of a region of interest using circular scans, wherein the detector area is optimally utilized for all source positions.
BACKGROUND
A system employing cone beam geometry has been developed for three-dimensional (3D) computed tomography (CT) imaging that comprises a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360 degree angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., “views”) along the scan path, the detector acquires a corresponding plurality of sequential sets of cone beam projection data (also referred to herein as cone beam data or projection data), each set of cone beam data being representative of x-ray attenuation caused by the object at a respective one of the source positions. The cone beam projection data is then processed to reconstruct a 3D image of the object using image reconstruction methods known in the art.
Various methods have been developed for 3D image reconstruction for cone beam x-ray imaging systems. For example, a back projection cone beam image reconstruction technique is described in U.S. Pat. No. 5,257,183, which issued on Oct. 26, 1993 to Kwok C. Tam, entitled “
Method and Apparatus for Converting Cone Beam X
-
Ray Projection Data To Planar Integral and Reconstructing a Three
-
Dimensional Computerized Tomography
(
CT
)
Image of an Object
”, which is incorporated herein by reference. This patent discloses a method and apparatus for converting cone beam data to values representing planar integrals on any arbitrary set of planes in Radon space for 3D image reconstruction through an inverse Radon transformation. Back projections can be mathematically accomplished for a cone beam source by inverse Radon transforming suitable planar integrals. The planar integrals are computed from detector integrals which utilize the measured cone beam projection data, i.e., the detected attenuated intensity representative of the density distributions of the object. The use of a cone beam source expedites data acquisition. A direct Radon inversion of three dimensional cone beam data from a cone beam source is not possible. Thus, before an inverse Radon transform can be undertaken in a three dimensional cone beam data implementation, the cone beam detector integrals must be reconfigured into planar integrals suitable for inverse Radon transformation. U.S. Pat. No. 5,257,183 discloses a method for image reconstruction by calculating Radon derivative data from the acquired cone beam data. The Radon derivative data is typically determined by calculating line integrals for a plurality of line segments drawn in the acquired cone beam data. Radon space driven conversion of the derivative data is used to develop an exact image reconstruction of a region-of-interest (ROI) in the object.
FIG. 1
illustrates a typical circular scanning geometry for three dimensional CT scanning employing cone beam geometry. An object
10
to be imaged is positioned within a field of view between a cone beam point source
11
and a two dimensional detector array
12
that acquires cone beam projection data. An axis of rotation
13
passes through the field of view and the object
10
. A midplane
14
is defined as the plane that (i) is normal to the axis of rotation
13
and (ii) contains the cone beam point source
11
. In the exemplary embodiments described herein, the axis of rotation
26
is the v axis, having its origin (0,0,0) at its intersection with the midplane. The coordinate system is fixed relative to the source
11
and detector
12
. When scanning the object
10
at a plurality of angular positions, the source
11
moves relative to the object
10
and the field of view rotates along a circular scanning trajectory
15
lying in the midplane
14
, while the detector
12
remains fixed with respect to the source
11
(or alternatively the object
10
can be rotated while the source
11
and detector
12
remain stationary). Data is acquired at a plurality of source positions during the scan. Data collected at the detector
12
represent line integrals through the object
10
. The approach to reconstruction then embodies calculating planar integrals on a set of planes from various line integrals through the object, then performing an inverse Radon transform on the planar integrals to reconstruct a three dimensional image of the object. It is known that data collected in such a single circular scan is incomplete and artifacts may be introduced into the reconstructed image.
One image reconstruction method that uses two circular scans and a Radon transform approach to three dimensional CT imaging is disclosed, for example, in U.S. Pat. No. 5,383,119, which issued on Jan. 17, 1995 to Kwok Tam, entitled “
Method and Apparatus For Acquiring Complete Radon Data for Exactly Reconstructing a Three Dimensional Computerized Tomography Image Of a Portion of an Object Irradiated By a Cone Beam Source
”, which is incorporated herein by reference. This patent discloses a method for imaging a region of interest (ROI) in a long object by scanning the ROI in 2 circular scan paths and a line scan that connects the two circular scans. The method enables exact 3D reconstruction of the image of a portion of interest of an object in the field of view of a cone beam source by selectively disregarding unwanted Radon data which may corrupt the imaging process and selectively recovering Radon data that would otherwise be lost. The method enables acquisition of a complete Radon data set through proper choice of scanning configuration and selective partitioning and manipulation of the acquired data.
More specifically, referring to
FIG. 2
, a ROI &OHgr;
o
within a long object
20
(e.g., human body) is reconstructed by cone beam data collected by (i) an upper scan trajectory
21
and lower scan trajectory
22
, which are taken to be spaced circular trajectories that are normal to the axis of rotation v, and (ii) a connecting line scan. A circular scan is performed for both the upper and lower bounds of the ROI and the data from the two circular scans are combined in such a manner as to appropriately reconstruct the ROI.
FIG. 3
depicts the method for combining the cone beam data from two circular scan paths for reconstructing the ROI in a long object, wherein cone beam source positions A and B correspond to the top and bottom scans, respectively. In general, processing the cone beam data for an exact image reconstruction involves filtering, either implicitly or explicitly, all line segments on the detector (i.e., the integration line segments). In the data combination process, at each cone beam view, the line segments being filtered are restricted to only the cone beam data bound by the angular ranges as depicted in FIG.
3
. In this manner, the totality of the cone beam data from all the contributing source positions covers every plane of integration intersecting the ROI in its entirety without any overlap.
More specifically, an image reconstruction method described in the above-incorporated U.S. Pat. No. 5,383,119 comprises the steps of performing a circular scan using a cone beam source along a circular path enclosing a ROI at the upper and lower extent o
Church Craig E.
F. Chau & Associates LLP
Kiknadze Irakli
Paschburg Donald B.
Siemens Corporate Research Inc.
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